1. Field of the Invention
The present invention relates to a three-dimensional tunnel memory device using as a memory element a multilayer Langmuir-Blodgett film wherein each layer can store charge.
2. Description of the Prior Art
A decrease in semiconductor integration orders has had considerable impact on the industrial fields, but micropatterning of semiconductor elements is not advancing. In conventional systems and fabrication techniques, devices of 0.1-.mu.m rule are regarded as having the smallest size. Under these circumstances, demand has arisen for large-capacity memories.
In order to satisfy such needs, extensive studies have been made recently on three-dimensional integrated circuits that are aiming at a high packing density, multifunctional performance, and high-speed operation.
For example, in the field of various recording media such as auxiliary memories, a three-dimensional recording medium is proposed by E. G. Wilson instead of a conventional two-dimensional one in order to achieve a high memory capacity and compactness. This memory utilizes as a memory element a multilayer Langmuir-Blodgett film, as described in document No. EP007135A1 or U.S. Pat. No. 4,534,015.
In general, an organic compound having both hydrophilic and hydrophobic groups can be expanded or developed as a monomolecular film on the water surface. Particularly, an organic compound having a hydrophilic group at one terminal and a hydrophobic group at the other terminal wherein the intensities of the hydrophilicity and hydrophobicity are the same (e.g., a soap), can be easily expanded to have a monomolecular thickness on the water surface with the hydrophilic group contacted with water. Such monomolecular films can be stacked on a substrate when the substrate is repeatedly moved across the expanded monomolecular film on the water, while maintaining a predetermined surface pressure. The resultant film is the multilayer Langmuir-Blodgett (LB) film.
The hydrophilic and hydrophobic groups can serve as potential barriers against electric charge, and the charge can be stored in a portion (i.e., a portion other than the hydrophilic and hydrophobic groups) of the monomolecules. Therefore, in a multilayer LB film, the charge can be stored in each monomolecular layer. In addition, the potential barrier constituted by the hydrophilic and hydrophobic groups has a height enough to allow tunnel hopping of the charge, and the charge can be transferred from on monomolecular layer to the adjacent monomolecular layer by an electric field. Using this principle, the charge can be stored in each layer, and information can be written or read by the electric field. A memory utilizing the above principle is the three-dimensional memory medium (device) proposed by E. G. Wilson.
FIG. 1 shows a schematic arrangement of this three-dimensional memory device. As shown in FIG. 1, the memory device includes multilayer LB film 11 wherein each layer can store or carry electric charges and therefore information, and means 12 for introducing charges into one side of the film in a time sequence corresponding to the information to be carried. The charge introducing means is located near the one side of the film 11, and constituted by, e.g., optical modulator. The memory device is also provided with means 13 for applying an electric field between the two faces of film 11 so as to transfer the charge from any layer to an adjacent layer, and readout means 14 for reading out a charge sequence stored by film 11.
In the memory device proposed by E. G. Wilson, when the magnitude of an electric field applied during tunnel hopping exceeds a given value, the charge is transferred in a state wherein the charge distribution in the memory medium is limited within a monomolecular layer of the LB film. However, since the tunnel hopping phenomenon is a probabilistic even with a "fluctuation", the probability distribution of the charge is spread along the transfer direction. From the probabilistic viewpoint, when the center of a memory charge pulse is transferred by a distance corresponding to m layers, the width of charge spreading is analyzed to extend as a Poisson distribution over .sqroot.m layers.
When the charge is transferred in a spreading state as noted above, the readout cannot be performed accurately.